There has been a great deal of research interest in recent years in laser sources that operate at mid-infrared wavelengths (i.e., longer than ˜2.5 μm). Compact, rugged, and reliable laser sources in this region are valuable in a number of applications, including infrared countermeasures, free-space communications, and remote sensing. In the past, parametric conversion of visible and near-IR lasers offered the best route to producing mid-IR sources, but recently, rare-earth doped, diode-pumped solid-state lasers have begun to emerge as a viable option. For such a laser to function, its host material must have a low phonon energy to prevent rapid multiphonon quenching of mid-IR lasing energy levels. The low phonon materials with the most promise for use in mid-IR solid-state lasers are the family of ternary alkyl lead halides, in particular, potassium lead chloride, KPb2Cl5.
At room temperature, KPb2Cl5 is monoclinic (nearly orthorhombic), with lattice parameters a=0.8831 nm, b=0.7886 nm, c=1.243 nm, and β=90.14°. It has a low maximum phonon energy of 203 cm−1, so it supports lasing out to at least 5 μm. It is hard enough to hold a polish (2.5 Mobs) and is only slightly hygroscopic, making it a good candidate for incorporation into practical laser systems. Single crystals of this material have been grown that incorporate trivalent rare-earth ions at concentrations up to ˜3%, replacing Pb2+ in one of its two distinct sites and creating a K+ vacancy for charge compensation. Multiple spectroscopic studies have shown the anticipated low rates of nonradiative quenching for their long-wavelength transitions. Laser action has been demonstrated in KPb2Cl5 in the near-IR at 1.06 μm (Nd3+) and 2.43 μm (Dy3+) and in the mid-IR at 4.5 μm (Er3+).
While there have been a large number of spectroscopic studies done on this material and many potential lasing transitions have been put forward, there have only been three laser demonstrations. This disparity between interest in the material and the number of successful lasers can be attributed directly to the difficulties involved in producing high-quality crystals. KPb2Cl5 will incorporate oxide impurities if it is melted under air, but melting it under vacuum causes it to decompose and leave behind metal impurities; this problem has been addressed successfully in the past through melting wider a chlorinating atmosphere. A more critical problem is the reproducible seeding of a Bridgman-Stockbarger growth. Even in an ampoule with a sharply-tapered tip, molten KPb2Cl5 has a strong tendency to supercool, sometimes remaining well below its freezing point for days at a time. This makes the formation of the tiny seed crystal needed for single-crystal growth an unreliable process. Additionally, the material's high degree of thermal expansion leads to the incorporation of a great deal of strain into the crystal as it cools, leading to cracking and reduced optical quality.